The present invention relates generally to a control apparatus for an induction motor. More specifically, the invention is concerned with an induction motor control apparatus suited profitably for controlling an onboard induction motor installed on an electric motor vehicle or car as a prime mover.
In order to ensure high operation performance and high speed for an electric car, not only independent driving of individual wheels is required but also a driving system inclusive of an electric motor and a control apparatus therefor must necessarily be implemented with a high reliability.
Under the circumstances, there arises in these years an increasing trend of using an inverter-driven induction motor for propelling the electric car in combination with a vector control system for the induction motor with a view to allowing the car to run at a high speed while affording highly accurate controllability.
As the prior art apparatuses of this type, there may be mentioned those disclosed in JP-A-56-66101, JP-A-57-177203, JP-A-63-87192 and JP-A-2-197284.
As to the driving system for the electric car, availability of such system is demanded which is capable of stopping or parking the electric car with safety in any situation and capable of restarting the car once stopped due to a fault, since occurrence of faults is practically inevitable.
In conjunction with this, in order to realize the vector control mentioned above, use of a rotational speed sensor (which may also be termed an angular speed sensor) is indispensably required. As a consequence, occurrence of a fault in the rotational speed sensor renders the vector control impotent, which in turn results in abnormal torque generation of the induction motor, incurring a dangerous state for the running or operation of the electric car.
Besides, in conjunction with the inverter for supplying electric power to the induction motor, it is also noted that the useful life of components constituting the inverter is also intrinsically limited, giving rise to occurrence of faults unavoidably. Of course, upon occurrence of a fault in the inverter, the control system will become impotent, to thereby make the driving system uncontrollable, incurring a dangerous situation as well.
However, in the prior art control/driving systems for electric cars such as those disclosed in the above-mentioned references, no adequate consideration is paid to the possibility of occurrence of dangerous states of the electric car due to abnormality of the rotational speed sensor and that of the inverter, and thus there are a variety of problems remaining unsolved, as enumerated below.
(1) For the purpose of controlling the speed and torque with high accuracy, the vector control is carried out by utilizing feedback of the output signal of the rotational speed sensor. Thus, the use of the rotational speed sensor is indispensably required for the vector control. Accordingly, upon occurrence of a fault in the rotational speed sensor, torque of the induction motor becomes excessively large or small or torque of minus polarity may be generated, which eventually leads to such unwanted situation where the stable current/power control is no more effective, making dangerous the operation of the electric car, presenting a serious problem.
(2) When a fault such as overcurrent, ineffectiveness of one of three output phases or the like takes place in the inverter which serves for supplying electric power to the induction motor, there may occur undesirable phenomena such as insufficiency of driving efforts or stoppage of the electric car and the like, which also leads to impotence of control/driving capabilities, incurring a dangerous running state of the electric car.
(3) Because the control system is susceptible to adverse influences of spurious signals such as external disturbance (noise), operation of the inverter is often stopped unnecessarily, giving rise to a problem that the control/driving system is lacking in reliability.
(4) In the case of a driving system for driving a plurality of individual wheels independent of one another for the purpose of realizing a highly accurate control of the car speed and the motor torque, occurrence of a fault in one of the driving systems brings about significant load unbalance relative to the other driving system or systems. Supposing, for example, that the driving power of a fault-suffering driving system becomes zero, generation of overall torque must be born by the other driving system(s). In another case where the driving power produced by one of the driving systems increases excessively, the torque produced by the other driving system will decrease. In any case, significant load unbalance prevails between or among a plurality of the driving systems, involving a problem that operation of the electric car inclusive of manipulation thereof by a steering handle becomes very dangerous.